CO Stretch Vibration Research Articles

Deep-Ultraviolet Photoexcitation of Aqueous Urea Forms Carbamic Acid/Carbamate in Less Than One Picosecond.

Urea is believed to have been essential to the synthesis of prebiotic nucleotides and thereby the RNA or DNA of the first lifeforms. Models suggesting that life began in wet-dry cycles around shallow aquatic ponds imply that reactants such as urea were exposed to deep ultraviolet irradiation from the young sun. Detrimental photodissociation of urea induced by deep UV excitation potentially challenges these models. We here follow the primary deep ultraviolet photochemistry of aqueous urea. The data show that urea is barely excited at 200 nm due to weak ultraviolet absorption. The likelihood of photodissociation is further reduced by strong intra-molecular coupling of the CN and CO stretch vibrations accompanied by an efficient dissipation of the excitation energy to the surrounding water molecules mitigated by urea-water hydrogen bonds. We find that 54±5 % of the excited urea molecules dissociate. Reactions between the photoproducts and surrounding solvent molecules form carbamic acid or the carbamate anions within 0.6 ps. The molecules that do not dissociate return to the electronic ground state in 2 ps. Interestingly, the photodissociation processes of urea in the aqueous phase is different from earlier reported reactions observed following the VUV photolysis of urea in noble gas matrices and highlight the potential influence of water on the prebiotic photochemistry.

Ligand and solvent effects on the catalytic activity and lifetime of zwitterionic Nickel(II) catalysts for alternating CO-Ethylene copolymerization

A series of zwitterionic nickel(II) compounds with anionic diphosphine ligands having varied substituents have been synthesized and studied as catalysts for CO-ethylene copolymerization. The electron-donating abilities of the ligands are assessed on the basis of the infrared absorptions of the CO-stretching vibrations of their Ni(0) carbonyl compounds. The equilibriums between Ni(II)-methyl and Ni(II)-acetyl and 4-coordinate and 5-coordinate Ni(II)-acetyls are studied using NMR and IR spectroscopies. With respect to the catalytic performances, a certain degree of steric hindrance around the Ni(II) center is required to achieve a high polymerization activity. The steric hindrance likely disfavors the 5-coordinate catalytically dormant state. Further increased steric congestion decreases the catalytic activity. The electron-donating ability of the ligand on the catalytic activity is modest within the relatively narrow range of variation of the present study. The catalyst lifetime and overall productivity are markedly improved by the presence of o-methoxy ancillaries on the phenyl group of the ligand although the o-methoxy decreases the activity of the catalyst. A comparison of the o-methoxy- and o-methyl-substituted ligands attributes the stabilization to the coordinating ability of the o-methoxy group rather than a steric or inductive electronic origin. The present catalysts are several times faster and more productive in THF than in toluene at room temperature. The increased activity is attributable to the coordinating ability of THF as opposed to polarity. The polymer products are strictly alternating copolymers of CO and ethylene.

Adsorption of Cd(II) Ion Using α-Cellulose Immobilized Humic Acid with Crosslinker Agent Epichlorohydrin

An environment is said to be polluted if there have been changes in the environmental order so that it is no longer the same as its original form, as a result of the entry and inclusion of a foreign substance or object in the environmental order. Various cases of heavy metal pollution have been reported in both developed and developing countries, as well as adverse effects on the population living in the vicinity. This heavy metal pollution including cadmium metal. Generally, the contamination of cadmium in waters originates from the waste of the metal ore processing industry. Cadmium which accumulates in the body of living things has a long half-life and generally accumulates in the liver and kidneys. This study focuses on reducing levels of heavy metals in the environment with an adsorbent from natural products, namely palm oil empty fruit bunches and humic acid from peat soils. This research combines two adsorbents of natural products, namely by immobilizing cellulose and humic acid with the epichlorohydrin crosslinker agent. The purpose of this study was to determine the effect of the optimum dose of epichlorohydrin on cellulose and humic acid immobilization, determine the optimum pH, adsorption isotherm, cadmium metal adsorption kinetics and determine the type of interaction between adsorbent and adsorbate. The results showed a link between cellulose and humic acid which was connected via epichlorohydrin from FT-IR results in certain wavenumbers, including OH vibration (3415cm–1), stretching CH vibration (2903 cm–1), NH bending vibration (1625 cm–1), COO stretch vibration (1373 cm–1) and CO stretch vibration (1058 cm–1). For the optimum dose of epichlorohydrin obtained at 15 mL (1 recipe) with an adsorption capacity of 7.4705 mg/g. While the optimum pH was obtained at pH 6, the adsorption isotherm obtained the largest capacity at 200 ppm by following the Freundlich isotherm (R2 = 0.9512).

Infrared spectroscopic and computational studies of Co(ClO4)2 dissolved in N,N-dimethylformamide (DMF). Vibrations of DMF influenced by Co2+ or ClO4− or both

Infrared (IR) spectroscopy for N,N-dimethylformamide (DMF) shows that the OCN bend (δOCN) and the CO stretch (νCO) vibrations undergo an upshift and a downshift, respectively, on the dissolution of Co(ClO4)2. Quantum chemical calculations are performed for optimizing the structures and predicting the IR spectra of model complexes for solute species. The calculations reveal that Co2+ exerts a much larger influence than ClO4− on the vibrations of DMF. For Co2+(DMF)6, in which each DMF molecule is coordinated to Co2+ via the O atom, the Co2+⋯DMF interaction upshifts the δOCN frequencies (+24 cm−1 on average) while the dipole coupling gives rise to splitting (12 cm−1) of the modes. On the other hand, the Co2+⋯DMF interaction downshifts the νCO frequencies (−15 cm−1 on average) while the splitting of the modes amounts to 37 cm−1. As a result, one of the νCO modes is located at an upshifted position (+13 cm−1) despite the O-atom coordination. For six-coordinated isomers of Co2+(DMF)7, the δOCN and νCO frequencies of the second-sphere DMF are close to those of bulk DMF in neat liquid. The calculations indicate that it is difficult to prove or exclude the formation of contact ion pairs [Co(DMF)5ClO4]+ and solvent-shared ion pairs [Co(DMF)6ClO4]+ by IR spectroscopy in the δOCN and νCO regions. However, asymmetric ClO stretches of the ClO4− moiety suggest that conceivable is the coexistence of solvent-shared ion pairs only.

Quasi-Static Two-Dimensional Infrared Spectra of the Carboxyhemoglobin Subsystem under Electric Fields: A Theoretical Study.

There is no doubt that electric fields of a specific frequency and intensity could excite certain vibrational modes of a macromolecule, which alters its mode coupling and conformation. Motivated by recent experiments and theories, we study the mode coupling between the Fe-CO mode and CO-stretch mode and vibration energy transfer among the active site and proteins in carboxyhemoglobin (HbCO) under different electric fields using the quasi-static two-dimensional infrared spectra. This study uses iron-porphyrin-imidazole-CO and two distal histidines in HbCO as the subsystem. The potential energy and dipole moment surfaces of the subsystem are calculated using an all-electron ab initio (B3LYP-D3(BJ)) method with the basis set Lanl2dz for the Fe atom and 6-31G(d,p) for C, H, O, and N atoms. Although the subsystem is reduced dimensionally, the anharmonic frequency and anharmonicity of the CO-stretch mode show excellent agreement with experimental values. We use the revealing noncovalent interaction method to confirm the hydrogen bond between the Hε atom of the His63 and the CO molecule. Our study confirms that the mode coupling between the Fe-CO mode and CO-stretch mode does not exist when the subsystem is free of electric field perturbation, which is coupled when the electric field is -0.5142 V/nm. In addition, with the increases of distance between the active site and the His92, there is no vibrational energy transfer between them when the electric field is 1.028 V/nm. We believe that our work could provide new ideas for increasing the dissociation efficiency of the Fe-CO bond and theoretical references for experimental research.

Direct Detection of Ultraweak CO Signal with Cavity Plasmon by Resonant Vibration–Plasmon Coupling

AbstractDirect, instant, and quantitative detection of molecules is highly desirable, however, optical detection of dilute gas molecules in the single molecular limit is extremely challenging. According to a recent experiment, a thin CO molecule layer will be absorbed at the edge of a metallic nanoparticle when the particle is exposed to CO environment; therefore, optical sensor becomes possible with such a layer. Here, a prototype plasmonic sensor for direct CO molecules detection based on the vibrational coupling to the surface plasmon polariton resonance in a finite metal/insulator/metal (MIM) nanocavity is proposed and investigated. Thanks to the ultrahigh spectral sensitivity of the cavity plasmon and its enhanced coupling to the CO stretch vibration, the single‐layered molecules in the cavity split the plasmon resonance in its optical spectra. The molecular signal is amplified by up to three orders of magnitude, giving a detection limit of ≈800 molecules, which lowers down the detection limit of molecule numbers by more than three orders of magnitude than the traditional surface plasmon‐based sensors. In the investigation, analytical vibration/cavity–plasmon coupling model developed herein agrees well with the numerical results. The study offers a promising way toward small molecule detection with plasmonic sensors.

Carrier mechanism of ZnO nanoparticles-embedded PMMA nanocomposite organic bistable memory device

ZnO nanoparticles embedded polymethylmethacrylate (ZnO-PMMA) organic bistable memory device was fabricated by sol-gel spin coating technique. ZnO-PMMA thin films were deposited on ITO coated glass substrates (Al/ZnO-PMMA/ITO). XRD and SAED patterns of ZnO-PMMA nanocomposite thin films showed the polycrystalline behavior. FTIR peaks of ZnO-PMMA nanocomposite thin films were observed at 540 cm−1 for Zn–O bond and at 1766 cm−1 for ester carbonyl CO stretch vibration of PMMA. The bandgap of ZnO-PMMA thin films was observed to be lower (3.07 eV) than the pure ZnO (3.20 eV), while the PL peak was observed at a wavelength of 403 nm. Current-voltage (I–V) measurements of the device, Al/ZnO-PMMA/ITO showed a nonvolatile electrical bistable behavior at room temperature. The ON to OFF current ratio of the bistable device was found to be ~104, which confirmed the write and the read sequence and discussed the carrier transport mechanism.

Influence of acetylation on physicochemical and morphological characteristics of pigeon pea starch

Native starches exhibit some technological drawbacks in industrial food applications associated with poor physico-chemical properties; however, modification of starch can remedy the lack of functionality. Starch was extracted from pigeon pea seeds, modified by esterification with acetic anhydride to obtain 0.05, 0.09 and 0.14 degree of substitution (DS). Acetylation decreased the heat transition temperatures and enthalpy of gelatinization. DS (0.05) starch showed the least retrogradation tendency (30.54%). FTIR spectra showed characteristic absorption peaks at 1245, 1370 and 1732 cm−1 (C–C, C–O and CO stretch vibrations, respectively). Integrated band intensities of all three characteristic FTIR peaks increased linearly with increasing DS. SEM imaging showed that acetylation did not lead to any observable change in starch granule shape and size; however, there was a significant rupture of starch granules at 0.14 DS. The physicochemical properties of the acetylated starches were significantly improved, demonstrating its potential usefulness as food additive.

Vibrational spectra, structure, theoretical calculations of 3-Fluoro-4-Hydroxybenzaldehyde: With evidence of hydrogen bonding

Fourier-Transform Laser Raman (3500-100 cm−1) and FT-IR (4000-400 cm−1) spectral measurements have been made for the solid 3-Fluoro-4-Hydroxybenzaldehyde. Geometry optimization and frequency calculations performed at two levels namely, RHF/6-31G* and B3LYP/6-31G* have yielded Cs symmetry with energies of −517.1388109 hartrees and −520.0240034 hartrees. A detailed interpretation of infrared and Raman spectra aided by the PEDs (potential energy distributions) for the computed frequencies has been reported. The PES scan yielded four distinct conformers out of which the most stable conformer is that with the orientation of CO of aldehyde group with respect to the hydroxyl group and fluorine in the cis form. The Fluorine substitution is seen to affect its structure and charge distributions in its vicinity as is seen by the NBO analysis and mulliken charges distribution.A broad IR band near 3130 cm−1, due to OH stretch shows evidence of hydrogen bonding. The aldehydic CH vibration predicted at 2801 cm−1 is observed at 2750 cm−1. Downshifting of the aldehydic CO stretch vibration predicted at 1725 cm−1 to an intense IR band at 1670 cm−1; increase in the frequency of in-plane bending vibration of OH as a shoulder absorption band at 1290 cm−1 from its predicted mode 1243 cm−1 are all further indications of OH⋯O bonding. Dimerization for the most stable conformer has been evaluated by B3LYP/6-31G* level. The dimer spectrum is seen to agree fairly well with the observed spectra. CF stretching vibration predicted at 1235 cm−1 is seen to couple with ring bending vibrations and is assigned to a strong Raman band at 1259 cm−1. The optimized structure and harmonic frequencies computed at B3LYP/6-31G* level are in good agreement with the experimental values.

Interplay between CN- Ligands and the Secondary Coordination Sphere of the H-Cluster in [FeFe]-Hydrogenases.

The catalytic cofactor of [FeFe]-hydrogenses (H-cluster) is composed of a generic cubane [4Fe-4S]-cluster (4FeH) linked to a binuclear iron-sulfur cluster (2FeH) that has an open coordination site at which the reversible conversion of protons to molecular hydrogen occurs. The (2FeH) subsite features a diatomic coordination sphere composed of three CO and two CN- ligands affecting its redox properties and providing excellent probes for FTIR spectroscopy. The CO stretch vibrations are very sensitive to the redox changes within the H-cluster occurring during the catalytic cycle, whereas the CN- signals seem to be relatively inert to these effects. This could be due to the more structural role of the CN- ligands tightly anchoring the (2FeH) unit to the protein environment through hydrogen bonding. In this work we explore the effects of structural changes within the secondary ligand sphere affecting the CN- ligands on FTIR spectroscopy and catalysis. By comparing the FTIR spectra of wild-type enzyme and two mutagenesis variants, we are able to assign the IR signals of the individual CN- ligands of the (2FeH) site for different redox states of the H-cluster. Moreover, protein film electrochemistry reveals that targeted manipulation of the secondary coordination sphere of the proximal CN- ligand (i.e., closest to the (4FeH) site) can affect the catalytic bias. These findings highlight the importance of the protein environment for re-adjusting the catalytic features of the H-cluster in individual enzymes and provide valuable information for the design of artificial hydrogenase mimics.

Probing specific oxides as potential supports for metal/oxide model catalysts: MgO(111) polar film

The growth of thermally evaporated magnesium oxide thin film on Mo(110) substrate in ultra-high vacuum was studied by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and work function (WF) measurements. It is shown that at a growth rate of c.a. 0.1 monolayer per minute and the substrate temperature of 600K the film acquires the MgO(111) structure. This structure begins to form at two monolayers and holds up to six monolayers. At higher thickness the film disorders due to weakening of the ordering effect of the isosymmetric Mo(110) support. Adsorption of CO and H2 on the formed MgO(111) film cooled down to 90K was studied by means of ultraviolet photoelectron spectroscopy (UPS) and reflection absorption infrared spectroscopy (RAIRS) and compared with in-situ obtained results for CO on Pt(111). Comparison of UPS data of CO on MgO(111) and Pt(111) in combination with RAIRS results reveals quite different bonding mechanisms on the metal and the oxide supports. The main feature of CO on MgO(111) is quite high intensity of CO stretch vibration, considerably exceeding that on amorphous MgO, and comparable to that of CO on Pt(111). This is presumably due to the electrostatic effect of the uncompensated microscopic dipole moment of ultrathin MgO(111) film on the enhancing of CO dynamical dipole moment. Adsorption of H2 dramatically reduces the CO stretch intensity as a possible result of removing of dipole moment of MgO(111) surface by hydrogen and (CO+H2) interaction.

FT-IR spectral and thermal characterization of ancient charcoals - A tool to support archeological and historical data interpretation

Recent, historical, and prehistoric charcoals were characterized using FT-IR spectroscopy and simultaneous thermal analysis to support interpretation of historical and archeological data. Chemical characteristics of aging play a dominant role in terms of the assignment of ancient charcoals. Different periods were differentiated according to the progressing aging process that becomes visible in the spectrum by the asymmetric and symmetric COO, the CH and the CO-stretch vibrations that can be assigned to carboxylates, the methyl- and the carboxyl-group. The increase of these components is paralleled by an increase of hydrophilic properties and therefore confirmed by the OH stretch vibration with a maximum at about 3400 cm−1. Charcoal from combustion residues was distinguished from charcoals produced for energy supply by means of infrared spectral features, classification and PLS-discriminant analysis. Apart from the spectral signature, the thermal behavior of combustion residues unambiguously reveals wood characteristics due to incomplete pyrolysis. Thermal profiles of combustion residues indicate mass losses at several temperatures according to the remaining wood components and a higher mass loss between 80 °C and 120 °C due to water evaporation.

Step-type and step-density influences on CO adsorption probed by reflection absorption infrared spectroscopy using a curved Pt(1 1 1) surface

In comparison to flat single crystals, the continuous variation of structure provided by curved crystals offers many benefits for the study of physical and chemical processes at surfaces. However, the curvature of the surface also creates experimental challenges. For infrared spectroscopy, in particular, adsorbates on metal samples are typically probed by grazing-incidence reflection-absorption infrared spectroscopy (RAIRS). In this geometry, a convex crystal acts as a strongly diverging mirror. The authors describe how the experimental difficulties introduced by a cylindrical surface can be resolved for RAIRS. A complementary mirror, placed directly downfield of the curved crystal within the vacuum chamber, minimizes the divergence created by the sample. By simply translating the infrared focus across the sample, the authors probe adsorbate vibrational spectra as a function of local step-type and step-density with high sensitivity and spatial resolution. Time-consuming sample exchange, and the concomitant sample-to-sample experimental errors, are eliminated. The authors apply this new technique to carbon monoxide adsorption on a curved Pt(1 1 1) crystal and use it to resolve the influence of step-type and step-density on the CO stretch vibration as a function of coverage.

FTIR and DFT studies of LiTFSI solvation in 3-methyl-2-oxazolidinone

Combined computational/FTIR spectroscopic analyses of 3-methyl-2-oxazolidinone (NMO) solutions with varying molar ratios of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are reported. Based on the second derivative spectral profile, overlapping peaks are distinguished as well as assigned to the vibrational modes of implicitly and explicitly interacting NMO molecules. Thereby, the geometry of a monomeric, a dimeric and a simplified solvation structure [Li(NMO)1]+ are optimized with a polarizable continuum model at a B3LYP theoretical level. With increasing contents of LiTFSI, the formation of Li+ solvation structures is scrutinized by semi-quantitative analysis of deconvoluted integral peak areas for three different ring-related vibrations and CO-stretch vibration. A discrepancy in the obtained data is observed implying the influence of the TFSI anion the ring-related vibrations are prone to. The solvation number of 4 is determined according to the CO-signal in diluted solution, which is proven by the computed Gibbs free energy for solvation of [Li(NMO)4]+ in a NMO medium (−41.7kcalmol−1).

Broadband two dimensional infrared spectroscopy of cyclic amide 2-Pyrrolidinone.

In the past one-and-a-half decade there has been a significant methodological and technological development of two dimensional infrared (2DIR) spectroscopy, which unfolds many underlying physical and chemical processes of complex molecules, especially for biological molecules. Due to the extreme technical difficulties and non-uniform performance of ultrafast laser, so far, the method has been mostly applied to a small spectral region. A rather simple experimental methodology is presented here which is able to cover a broad spectral range from 1500 cm(-1) to 3500 cm(-1) to explore the molecular structure of cyclic amide, 2-Pyrrolidinone, via the time-resolved coupling of CO, CH and NH stretch vibrations. The signature of the coherent as well as incoherent coupling has been found. The amide-I band is incoherently coupled to CH and NH stretch vibrations and acts as an acceptor mode for vibrational energy relaxation from CH and NH stretch vibrations.

Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation

Vibrational spectroscopy using sum-frequency generation has been used to investigate the coupling between a ferromagnetic thin film and adsorbed molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a strong magnetic contrast with a pronounced temperature dependence, underlining the high sensitivity of this adsorbate-specific spectroscopy method. Our results indicate that the strong temperature dependence is caused by dynamical changes in the surface chemical bond when the CO stretch vibration is coupled to thermally excited external vibrational modes.

Experiment-Based Kinetic Monte Carlo Simulations: CO Oxidation over RuO2(110)

Kinetic Monte Carlo (kMC) simulations of the CO oxidation over RuO2(110) have been performed for a variety of different reaction conditions ranging from 10–7 to 10 mbar and temperatures from 300 to 600 K. The kMC simulations are based on reaction rates of elementary steps including diffusion, adsorption/desorption, and recombination of surface CO and O. The activation barriers for these elementary processes are mostly taken from temperature programmed reaction and desorption experiments of well-defined coadsorbate layers on RuO2(110) under ultrahigh vacuum conditions. We show that the experimental kinetic reaction data under steady state reaction conditions both in the 10–7 mbar range and in a batch reactor up to 10 mbar are reconciled within this experiment-based kMC approach. Experimental in situ reflection absorption infrared (RAIR) spectra in the frequency range of the CO stretch vibration depend sensitively on both the adsorption site and the local environment of the CO molecules, encoding thus the s...

The infrared spectrum of NN⋯CO+ trapped in solid neon

Codeposition of a Ne:N(2):CO = 200:1:1 mixture at 4.3 K with a beam of very pure neon atoms excited to their energy levels between 16.6 and 16.85 eV leads to stabilization in the resulting solid of sufficient NNCO(+) for detection of its NN- and CO-stretching vibration fundamentals. Detailed isotopic substitution studies and density functional calculations for the various isotopologues support the identification of NNCO(+) and permit estimation of the positions of two of its low-frequency fundamentals. A sufficient concentration of NOCN is also stabilized in the neon matrix for detection of its NO-stretching vibrational fundamental.

Methanol clusters (CH3OH)n, n = 3–6 in external electric fields: Density functional theory approach

Structural evolution of cyclic and branched-cyclic methanol clusters containing three to six molecules, under the influence of externally applied uniform static electric field is studied within the density functional theory. Akin to the situation for water clusters, the electric field is seen to stretch the intermolecular hydrogen bonds, and eventually break the H-bonded network at certain characteristic threshold field values of field strength in the range 0.009-0.016 a.u., yielding linear or branched structures with a lower energy. These structural transitions are characterized by an abrupt increase in the electric dipole moment riding over its otherwise steady nonlinear increase with the applied field. The field tends to rupture the H-bonded structure; consequently, the number of hydrogen bonds decreases with increasing field strength. Vibrational spectra analyzed for fields applied perpendicular to the cyclic ring structures bring out the shifts in the OH ring vibrations (blueshift) and the CO stretch vibrations (redshift). For a given field strength, the blueshifts increase with the number of molecules in the ring and are found to be generally larger than those in the corresponding water cluster counterparts.

Quantum Cascade Laser Spectroscopy and Photoinduced Chemistry of Al–(CO)n Clusters in Helium Nanodroplets

Helium nanodroplet isolation and a tunable quantum cascade laser are used to probe the fundamental CO stretch bands of aluminum carbonyl complexes, Al-(CO)(n) (n ≤ 5). The droplets are doped with single aluminum atoms via the resistive heating of an aluminum wetted tantalum wire. The downstream sequential pick-up of CO molecules leads to the rapid formation and cooling of Al-(CO)(n) clusters within the droplets. Near 1900 cm(-1), rotational fine structure is resolved in bands that are assigned to the CO stretch of a linear (2)Π(1/2) Al-CO species and the asymmetric and symmetric CO stretch vibrations of a planar C(2v) Al-(CO)(2) complex in a (2)B(1) electronic state. Bands corresponding to clusters with n ≥ 3 lack resolved rotational fine structure; nevertheless, the small frequency shifts from the n = 2 bands indicate that these clusters consist of an Al-(CO)(2) core with additional CO molecules attached via van der Waals interactions. A second n = 2 band is observed near the CO stretch of Al-CO, indicating a local minimum on the n = 2 potential consisting of an "unreacted" (Al-CO)-CO cluster. The line width of this band is ∼0.3 cm(-1), which is about 30 times broader than the transitions within the Al-CO band. The additional broadening is consistent with a homogeneous mechanism corresponding to a rapid vibrational excitation induced reaction within the (Al-CO)-CO cluster to form the covalently bonded Al-(CO)(2) complex. Ab initio CCSD(T) calculations and natural bond orbital (NBO) analyses are carried out to investigate the nature of the bonding in the n = 1, 2 complexes. The NBO calculations show that both π-donation (from the occupied aluminum p orbital into a π* antibonding CO orbital) and σ-donation (from CO into the empty aluminum p orbitals) play a significant role in the bonding, analogous to transition-metal carbonyl complexes. The large red shift observed for the CO stretch vibrations is consistent with this bonding analysis.